The present invention relates to a semiconductor integrated circuit of a CMOS structure including a plurality of static random access memories (SRAMs) and a plurality of logic circuits respectively accessing these SRAMs, and a method for fabricating the same.
A recent system LSI developed for portable equipment such as cellular phones includes a digital signal processor (DSP) required to perform high-speed processing of voice and moving image data and a central processing unit (CPU) responsible for processing of applications and system control during standby. A high-speed SRAM is necessary for the DSP as a buffer memory for transmit/receive data. Also, a SRAM is necessary for the DSP to be used as a work memory.
In conventional system LSIs for portable equipment, the threshold voltage (Vt) of MOS transistors tends to be set comparatively high for reduction of leakage current during standby. In such system LSIs, also, it is required to maintain the speed of the DSP and the CPU. Therefore, tight transistor design and circuit design are conventionally made in the tradeoff between the speed and the leakage current. In reality, processes specified for portable equipment, including increasing the threshold voltage slightly and increasing the gate length of MOS transistors slightly for avoiding a variation in leakage current, have been developed, separately from standard processes for non-portable equipment.
With the connection of portable equipment such as cellular phones to the Internet, the data processing amount of the portable equipment has sharply increased. In addition, the capacity of a memory for buffering data of moving images and data from the Internet has also increased.
In the situation described above, increase in leakage current due to the increased memory capacity has arisen concurrently with the request for high-speed operation of the DSP and the CPU. For solving these problems, the level of optimizing the transistor design and the circuit design as described above is no more useful.
The problem of reducing the leakage current may be solved by shutting off the power to circuits inside the LSI. However, since portable equipment, which is connected to a wireless network, has processing to be processed during standby, it is no more possible to simply shut off the power.
In the future, in mounting a number of circuit blocks having different uses on one chip, if fabrication processes optimized for the respective circuit blocks are individually adopted, the entire fabrication process will be complicated and thus the fabrication cost will increase.